Anion Equilibria and Stability of Quaternary Phosphonium Chloroaluminate Ionic Liquids

Abstract

The anion equilibria and thermal stability in a series of quaternary trialkyloctylphosphonium chloroaluminate ionic liquids are investigated qualitatively by Raman spectroscopy, broadband dielectric spectroscopy, and nuclear magnetic resonance spectroscopy. Results indicate that Lewis acidic chloroaluminate ionic liquids based upon the phosphonium cation are less stable than those based upon other molecular cations. A slow time-dependent drop in room-temperature dc ionic conductivity as well as a step drop at elevated temperatures is tentatively attributed to a reaction involving the Lewis acidic Al2Cl7 - anion and the phosphonium cations. This reaction results in significant changes in the 1H, 13C, and 31P NMR spectra, but only minor changes in the Raman spectra. Furthermore, extended time at elevated temperatures results in a significant quantity of neutral aluminum chloride sublimating from the liquid. These results demonstrate an unusual reactive instability of phosphonium-based Lewis acidic chloroaluminate ILs.